Defense - 540 for any raid is the minimum.
Hit - Though helpful for threat its not critical, take what you get in the form of equips on your gear. You can use the hit food to help off set this and run glyph of Taunt on encounters where it is needed for proper tank swaps. There is a massive lack of hit on most ToC gear, there are some options from 10 man Heroic ToC. Most tanks have done just fine with as little as 20-40 hit rating regardless. ICC gear has more items with hit.
Expertise - 26 skill, which meets the "soft cap" commonly called the "dodge cap" is all you really need. If you can increase your expertise beyond that with a minimal loss to survival it generally helps some to reduce parry gibs. However in ICC this seems to be less of an issue so far.
Mitigation - More armor the better for physical encounters. It means nothing against magic damage.
Avoidance - Have a set for fights that would benefit more avoidance over EH (stamina and armor). Keep in mind at higher levels of avoidance diminishing returns come into effect and reduce how much you get per point. Generally speaking you get plenty of avoidance in the form of equips on your gear naturally as you gear up. Which is the reason we have Chill of the Throne in ICC.
Overall - A good tank should have several different sets of gear, even if that means only swapping a trinket/ring/neck to make a different set. There are no "set requirements" to speak of other than 26 expertise skill and 540 defense.
I have 2 sets currently:
- Max HP set consisting of my best Health items for fights with higher magic than physical damage.
- Max EH consisting of my highest Armor/Stamina items for everything else.
I plan on building an Avoidance set consisting of the T10 gear, but that's a lower priority since hard modes are not out yet in ICC and thus far I have not seen any fights that Avoidance outweighs EH. Though I am sure some will argue Gunship and Saurfang.
A good gauge is to acquire as many items as possible from the previous tier in order to be ready for the next tier of content. Keep in mind that though gear is important, skill and ability to use that skill are much more important.
I will use this quote to illustrate this point:
The reality is, 99% of the WoW population always out gears the content they do. There’s no better indication of this than the world first raid guilds that always are getting their kills with worse gear than you’re using. Which isn’t to say you don’t need any gear, of course you do, but it is to say that it’s rarely gear that’s going to hold you back.